Conventionally, core drills made by combining a center drill and a tubular core body have been widely used. The core body has a drill cutting blade at a lower end opening edge portion. In accordance with the core drill, while the center drill is forming a center hole first, the core body can subsequently form a large-diameter hole.
As the core drill of this type, there is a core drill in which a base body including a shank and a core body having a drill cutting blade at a lower end opening edge portion are constructed as separate parts. Various constructions in which the core body detachably attached to the base body is locked by a connection holding means have been proposed (see Japanese Unexamined Patent Application Publication No. 2000-326126.)
An attachment structure of the core body with respect to the base body of the core drill described in Patent Document 1 includes an attachment plate provided on a lower end of the base body including the shank. A core body attachment portion is provided on a lower portion of the attachment plate, and hook-like concave engagement grooves are formed at a plurality of positions on the outer peripheral surface of the core body attachment portion. A tubular connection opening portion into which the core body attachment portion of the base body is fittingly inserted is formed at an upper end opening of the core body. On the inner surface of the connection opening portion, engagement projections are formed, which engage with and disengage from the engagement grooves of the core body attachment portion. When attaching the core body to the base body, the core body attachment portion of the base body is fittingly inserted into the connection opening portion of the core body, so that the engagement projections engage with end portions of the engagement grooves. Then, the core body at this position is locked by the connection holding means attached to the base body.
In accordance with the core drill thus constructed, the base body and the core body can be connected to and separated from each other by simple operations. Therefore, the core drill is convenient in that, for example, after having formed a cut hole on an object to be drilled, such as a concrete material, using an electric drill to which the core drill is applied, cut masses remaining in the core body can be easily removed by separating the base body and the core body from each other. Moreover, after attaching the core body to the base body, the connection holding means attached to the base body can fix the core body and the base body in a state where the base body and the core body are surely connected to each other. Therefore, loosening, etc. between the base body and the core body do not occur during the drilling operation.
The cut hole formed on concrete or the like by using the core drill is used, for example, to allow a pipe or the like having a size corresponding to the diameter of the cut hole to be inserted therethrough. However, an opening edge of the cut hole formed on concrete or the like by using the core drill is square. Therefore, to facilitate the insertion of an end portion of the pipe into the cut hole, it is necessary to chamfer the opening edge. Moreover, to allow a bolt or a screw to be inserted into the cut hole, it is necessary to form on the opening end portion a countersunk portion having a size capable of accommodating a head portion of the bolt.
However, at a worksite, for example, manually chamfering, by a worker using a cutting tool, the opening edge of the cut hole formed on concrete or the like by the core drill takes a lot of time and labor, and this deteriorates the work efficiency and personnel expenses increase, thereby leading to the increase in cost. Therefore, the accuracy improvement of the finished chamfered portion may not be realized.